1. Field of Use
This invention relates generally to register control systems for web-processing apparatus, such as multicolor rotogravure printing presses which have a plurality of color decks.
In particular, it relates to register control systems for maintaining a constant actual base repeat length at the key color deck thereby reducing or eliminating key color repeat length errors.
2. Description of the Prior Art
The fields of publication, packaging, and newspaper printing demand high quality color registration. Each color must be accurately applied to a specific spot on the moving web. Prior art automatic register control systems aim to reduce mis-register waste caused by small or large errors which were previously detectable only after the final product was printed. A typical register control system inspects a moving web in a printing press and regulates servo motors to vary web length and tension and to maintain precise registration between color units. In a typical multicolor printing press the first significant color cylinder (usually yellow) at the key color deck prints a small register mark on the web along with each illustration. The marks are scanned at subsequent printing decks on the press to detect variations in web length. Under tension, the web elongates in various degrees, dependent upon moisture content, caliper, cylinder sizes, dryer temperature, and any manual and automatic adjustments made to the press during a run.
To correct for mis-register, there are two types of compensation commonly used on presses--web compensation and cylinder compensation. On a press using cylinder compensation, the printing cylinder is rotated to change the location of the printing between printing decks. On a press using web compensation, a movable compensator is operated between the printing decks to change the length of web between two decks. As web length changes between decks (increases or decreases), the position the cylinder prints downstream of the preceding deck advances or retards. To advance the printing refers to movement of the printing towards the exit end of the press; to retard refers to movement away from the exit end of the press.
Publication gravure, letter-press, offset and packaging gravure presses sometimes use cylinder-to-mark register control systems. Such a system receives part of its information from a register mark printed on the web and part of its information from a reference mark generated by or directly related to print cylinder position. The reference mark, generated once per cylinder revolution, is transmitted to a computer in a register control wherein it is compared to the arrival of a scanner signal produced by the register mark. An encoder provides for machine time increments, for example, and sends out a series of 20,000 pulses or digits per cylinder revolution. Signals based on the reference mark and the scanner signal should arrive simultaneously. Any difference measured by one or more encoder pulses is counted and sent out as a registration error signal to operate or adjust the compensator. Accuracy of a register control system is a function of the number of divisions into which the printing repeat length is divided by the encoder and in this example is 1/20,000 of the repeat length. Register control systems are used with presses which operate at speeds up to 2500 feet of web per minute.
It is known that the majority of all intercolor register errors in rotogravure printing is caused by changes in the actual print repeat length in the first or key color deck and the response of sebsequent colors to this error. Prior art printing register control systems as hereinbefore described attempt to sequentially match intermediate colors to the actual repeat of the first or key color. It is also known, however, that maintenance of the desired actual print repeat length in the first or key color deck depends on maintaining the tension level appropriate to the condition of the web material being printed.
Variation of actual print repeat length of the first or key color may be caused by either one of two variables; first, a variation of the tension going into the first or key color deck caused by tension transients in the web introduced from the unwind roll, butt splicer, infeed control, speed changes in web speed, and second, variations in the web material being printed, such as moisture content or board (web) caliper. All of the above-mentioned variables, when introduced into a press operating in a steady state condition, require the establishment of a new infeed web tension level rather than reestablishment of a constant preset tension in order to maintain a constant repeat length of the key color.
Thus, print register errors can occur in the key color deck that are permanent in nature, that is, they represent a change in the print repeat length. Thus, change in repeat length is caused by changes in the web physical characteristics, i.e., moisture content, caliper, elasticity, etc., and is most noticeable across a roll splice. Key color repeat length changes can also occur on a gradual basis throughout a web roll as the web characteristics change between the outermost wraps and the core of the web roll.
Primary register control systems, as hereinbefore described, responding to intermittent and cyclic print errors, operate to maintain a printed register mark applied by the key color deck print cylinder in a fixed position relative to a subsequent printing cylinder reference position and thus do not correct for a fixed change in key color repeat length. The resulting register error at each printing deck after the first color is referred to as a phase error. Test analysis has shown that this phase error magnitude is not a constant for each printing deck, and may be as high as 0.030 of an inch for one color. Corrections for phase error on presses equipped only with a primary register control system were heretofore made manually. Unfortunately, manual corrections require greater press operator attention and are made "after the fact," that is, when the misregistered blanks are on the delivery table and the press is loaded with misregistered web.
U.S. Pat. Nos. 2,082,705 and 3,025,791 are prior art examples of web processing apparatus wherein infeed web tension is controlled automatically in response to marks on the web. However, the prior art does not teach repeat length or key color controls which take into account the fact that repeat length changes sometimes represent more than transitory changes and, in fact, are actually permanent changes which require a new or different set of parameters within which repeat length errors of a different magnitude are to be sensed and corrected for by establishment of a new infeed tension level.